ares/comnet assuming that apfourmaux/ares/ares18x2-en-2.pdf · provide routerx’s routing table...

Master Informatique 1styear 1stterm Anonymous number:

Ap

ARes/ComNet — 2018-2019

Final exam : Version A in English

Duration: 2h00

Allowed: One handwritten A4 sheet (front and back)

Forbidden: All other documents, pocket calculators, mobile phones, etc.

Ap

The exam consists of 3 sheets recto/verso, including the questions and spaces in which to write your answers. You must hand

back only these sheets at the end of the exam. To ensure anonymity, you must write the anonymous ID number assigned to you

on each sheet (you must not write your name on the exam).

You must write your answers in the frames provided for that purpose.

1 Application Layer (6.5 points)

In this exercise, we consider only the application layer protocols and only the messages that they exchange at this

abstraction layer.

1. Two universities are considered, each o↵ering an e-mail service accessible from several synchronized e-mail clients and via

web-mail. Specify the application protocols that will be used. Justify your answers.

In your following answers, use numbered arrows to display the application layer exchange sequences required for each of the asked

user actions. For each arrow, specify in addition the protocol and the important parameters of the request or the answer. Use the

remaining space to justify all your answers.

Sorbonne U. and Tokyuo U. each have: a domain name (.su.fr or .tu.ac.fr), an authoritative name server NS ... for external

resolutions to this domain name; a local name server NS loc... for local resolutions; a mail server Mail ... for SMTP and IMAP;

and a web server Web ... . Local host name resolutions are directly performed by the local name server. This server initially has

empty external information caches.

2. The Sorbonne University user A would like to send an e-mail to his colleague B from Tokyo University. Describe the

exchanges.

NS rootNS .fr

Sorbonne U.

NS .su.fr

NS loc SU

Mail SU

Web SU

A

NS .jpNS .ac.jp

Tokyo U.

NS .tu.ac.jp

NS loc TU

Mail TU

Web TU

Internet

1/12 Version X2-2018-en-vA-b

Master Informatique 1styear 1stterm

3. The user B from Tokyo University is in Paris to work with his colleague A. His computer is connected on the same network

as A. To retrieve his messages, B uses the Tokyo University web-mail. To exchange a results file, these two users use e-mails.

Assuming that A has just sent this file as an attachment with a short text (following the procedure described in the previous

question), explain the necessary exchanges to display the text of the e-mail on the web-mail client of B.

NS rootNS .fr

Sorbonne U.

NS .su.fr

NS loc SU

Mail SU

Web SU

A B

NS .jpNS .ac.jp

Tokyo U.

NS .tu.ac.jp

NS loc TU

Mail TU

Web TU

Internet

4. As a result of the previous exchanges, the user B now wants to recover the file as an attachment of the message from which

it initially received only the text part. Several solutions are possible. Specify your assumptions and describe the exchanges

in this context.

NS rootNS .fr

Sorbonne U.

NS .su.fr

NS loc SU

Mail SU

Web SU

A B

NS .jpNS .ac.jp

Tokyo U.

NS .tu.ac.jp

NS loc TU

Mail TU

Web TU

Internet



Tra



Duration: 2h00



Tra


back only these sheets at the end of the exam. To ensure anonymity, you must write the anonymous ID number assigned to you

on each sheet (you must not write your name on the exam).


2 Transport layer (6.5 points)

1. Describe why an application developer might choose to run an application over UDP rather than TCP.

2. Answer with True or False and justify your answer.

(a) The size of the TCP advertised window never changes throughout the duration of the connection.

(b) Host A is sending to host B a large file over a TCP connection. Assume host B has no data to send to A. Host B will

not send acknowledgements to host A because host B cannot piggyback the acknowledgement on data.

(c) Suppose Host A is sending a large file to Host B over a TCP connection. If the sequence number for the current

segment is m, then the sequence number for the subsequent segment will necessarily be m + 1.

(d) Suppose Host A sends one segment with sequence number 38 and 4 bytes of data over a TCP connection to Host B.

In this same segment the acknowledgment number is necessarily 42.

Congestion Control is the sender sensing congestion on the network by timing ACKs and controlling its sending rate. In the

following,A cwindA stands for Congestion Window andA ssthresholdA stands for Slow Start Threshold.

c) When TCP is in slow start, what is the result of receiving 2 ACKs before timeout regarding cwind value ?

d) What happens when cwind value reaches ssthreshold ?



Consider a communication between a host A and a host B with the following assumptions:

• RTT (Round Trip Time): round trip time between A and B is constant and is equal to 10 ms.

• No congestion.

• AWnd: The Advertised Window size is equal to 24 KB.

• MSS (Maximum Segment Size): The maximum size of the segment is 2 KB.

e) How long does it take before the first full reception window AWnd can be sent?

f) Suppose the TCP congestion window cwind is set to 18KB and a timeout occurs. What would be the size of cwind for the

next five transmissions ? Assume that the maximum segment size is 1KB.

g) What is the global synchronisation problem in TCP congestion control and why does it occur?



Net



Duration: 2h00



Net


back only these sheets at the end of the exam. To ensure anonymity, you must write the anonymous ID number assigned to you.


3 Network Layer (7 points)

A company has the following network:

LAN A31 hosts

LAN B30 hosts

LAN C28 hosts

X Y

Z

H

1. You are the network administrator. Allocate IPv4 addresses to each of the subnetworks from a block that starts at 86.3.128.0.

Do this in order from the subnet that has the largest number of hosts to the smallest. Fill in the following table:

Subnet name Network addressPrefix length Netmask Broadcast address

. . . / . . . . . .

. . . / . . . . . .

. . . / . . . . . .

. . . / . . . . . .

. . . / . . . . . .

2. How large a prefix do you require for this network? Propose the most compact solution and justify your answer.



3. Provide the routing table of host H located in LAN C, assuming that the host’s network interface is called i1.

destination mask gateway interface

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

4. Provide router Z’s routing table (interfaces i1, i2, and i3).


. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

5. Provide router X’s routing table (interfaces i1, i2, and i3; gateway address to the internet: 80.0.2.24 through a /30 prefixed

network).


. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .


Don’t hand back this sheet!




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